As a cooler arranged in an intake system (intake air passage) of an engine, an intercooler to cool an intake air to be introduced into the engine, for example, is known. As a cooler arranged in an exhaust system (exhaust gas passage) of the engine, an EGR cooler disposed in an exhaust gas recirculation system (hereinafter referred to as “EGR system”) adapted to partially recirculate an exhaust gas into the intake system, for example, is known.
A significant reduction in cooling efficiency of these coolers may affect an engine performance. Accordingly, a technique of arranging a temperature sensor on a downstream side of the cooler, calculating the temperature of a fluid on an upstream side of the cooler based on, for example, a state quantity of the fluid, and then comparing a value obtained by the sensor to the calculated value to diagnose the cooling efficiency of the cooler has been proposed (see, for example, Patent Literature Documents 1 and 2).